A vacuum pump may be used in what is commonly referred to as a “house” vacuum system whereby a single vacuum pump or a number of vacuum pumps are used to draw fluid from a number of different enclosures, often serving a number of different laboratories. Consequently, the fluids evacuated by such a house vacuum system can be of indeterminate composition and concentration. Fluids that may not pose any particular hazardous threat when pumped in isolation may, when mixed with similarly innocuous substances, generate hazardous, potentially flammable atmospheres when mixed together inside the vacuum pump.
If the ratio of the flammable fluid mixture to oxidant within the pumped fluid, hereinafter referred to as the concentration of the pumped fluid, lies between certain limits, namely the upper and lower explosion limits (usually expressed as the percentage by volume in air) then combustion will take place if a source of ignition is present.
It is possible to mitigate against a flammable atmosphere being generated within the vacuum pump by introducing quantities of purge gas into the vacuum pump, which dilute the potentially hazardous mixture contained therein. The dilution ensures that the concentration of the pumped fluid remains below the lower explosive limit associated with that particular fluid mixture. If the vacuum pump is evacuating by-products from a known process, an appropriate level of purge gas required may be readily discernable. However, in some circumstances, for example in a house vacuum system, the pumped fluid is of indeterminate composition and consequently significant quantities of purge gas may need to be introduced into the pump to ensure safety requirements are met at all times. The use of purge gas leads to an increased cost of consumables and can be detrimental to the effectiveness of the pumping process. By introducing significant quantities of purge gas into the vacuum pump, the volume of fluid to be transported through the vacuum pump increases significantly. Consequently, a larger capacity pump needs to be provided to accommodate the increased volume of fluid. Disadvantages associated with using a higher capacity pump are increased capital costs, increased volumetric footprint and increased power requirements.
It is therefore desirable to provide a means of detecting a flammable atmosphere, independent of the particular composition of the fluid mixture, to enable techniques for mitigating against the formation of a flammable atmosphere within a device, for example a house vacuum pumping arrangement, to be used more effectively and efficiently. In one example, the extensive use of purge gas may be avoided.